The invention relates to heat exchanger tubes and particularly to such tubes which are provided with fins. Finned tubes are used extensively in applications such as refrigeration and processing where it is desirable to maximize surface contact area and minimize tube length, weight and volume.
Materials for heat exchanger tubing vary widely depending upon their characteristics such as cost, corrosion resistance and fabricability. In recent years, titanium has been receiving increased usage due to its excellent corrosion resistance in a variety of environments as well as due to its increased availability and decreased cost of welded tube relative to the extruded seamless tube formerly used. However, fabrication of finned tubing out of titanium is severely complicated by some differences in the mechanical and physical properties of titanium as compared with other materials, notably copper, aluminum and various nickel alloys. Most significant of these properties is the rate of work hardening. When metal is worked at a temperature below its crystallization temperature, its strength is increased while its ductility or ability to be deformed without cracking is decreased. Continued deformation in this region of temperature can continue until a point is reached where fracture occurs. This fracture may be complete separation of the part into two or more pieces. However, such total separation is usually preceded, except in the most brittle of metals, by localized cracking. In a normal production operation, it is desirable to establish conditions such that any form of cracking rarely occurs. Thus, to take into account the many variables involved in a finning operation, such as tool wear and variations in dimensions, material and temperature, a total deformation significantly below the values determined by destructive tests is chosen.
There are several alternative methods of improving the workability of material so as to increase its ability to accommodate more deformation without failure. These include increasing the working temperature of the material and heat-treating material between successive stages of deformation. As a general rule, the strength of a material decreases, and the ductility increases, with increasing temperature. However, with most metals and alloys, a point is reached, as temperatures increase, at which the material no longer work hardens. As rapidly as the material is deformed, the metal relieves itself of the effect of the strain and a new strainfree, non-work-hardened structure is generated.
Heat-treatment is a broad term which covers any heating operation performed on a metal and its effects of course vary with each metal or alloy. Recrystallization is the heat-treatment of most significance in the present context. During recrystallization, old grains, which have accommodated deformation and, consequently, have become strain-hardened, are replaced progressively through the formation of new grains which are free of the effects of the previous strain and are thus ready to accommodate as much strain as were the original grains before any deformation occurred. Another heattreatment, known as recovery, involves the reduction or removal of work-hardening (strain-hardening) without apparent, or at least major, motion of grain boundaries, that is, without major recrystallization. Recovery will usually result in the ability of a metal to accept some more deformation prior to fracture, but not as much as would have been accommodated had the material been fully recrystallized. While high temperature working and heat treatments do offer some technical advantages, they are usually accompanied by increased costs due to increased equipment, labor, facility and other associated components of manufacturing.
In the manufacture of finned tubing, the fins usually extend in a helix along the length of the tube and are produced through use of forming tools which deform the tube and force a portion of the metal radially outwardly to form fins while at the same time the I.D. of the tube is forced radially downward. The tools produce a continuous fin which normally has an outside diameter equal to or slightly less than the starting outside diameter of the tube. Between each fin is a groove which is formed by the toolilng and which defines the root diameter (R.D.) of the fin. The R.D. is smaller than the original diameter of the tube. In the conventional fin-forming operation, the forming is done using one or more sets of discs which force the tube against an internal mandrel pin which has a work surface with a constant diameter which is less than the I.D. of the starting tube. During the deformation, the amount of work-hardening present in each portion of the work piece will vary widely. For example, there will be areas of high work-hardening near the outer diameter of the fin, with relatively low work-hardening effect in the tube wall under the fin. If one then assumes that the areas of highest work-hardening which are produced near the outer diameter of the fin are the maximum achievable prior to failure, one may conclude that this configuration limits the fin dimensions which are possible without use of hot working, heat treatment, and/or metal removal procedures.
Historically, in the development of finned tubing, fin counts and fin heights started with lower fin densities, such as 16 fins per inch (f.p.i.) and higher fin heights, such as 0.050", especially in the easy to fin materials such as copper, copper alloys and low carbon steels. Most probably, this situation prevailed more because of the ability of manufacturers to fabricate suitable durable tools than because of the ability or inability of the material to withstand the work applied. At the present time, advances in tooling and in finning technology have allowed manufacture of products with fin densities of double or more the aforesaid figure of 16 f.p.i. In the case of the easier to fin alloys, the prior fin heights have been held and even advanced to 0.060" or so. Obviously, the general goal of development work in connection with fin tubes is to maximize heat transfer while minimizing tube length and cost. Where higher fin counts and higher fin heights can be achieved, it is obvious that the ratio Ao/Ai of the outside area to the inside area will be increased, thus increasing heat transfer and permitting less length of tubing to be used than is the case with a lower Ao/Ai ratio.
In the situation of the difficult to fin refractory alloys such as titanium and stainless steel, it had been felt necessary, in the past, to have fin walls under the fin, for titanium, of about 0.042" to produce fin densities of about 19 f.p.i. and fin heights of 0.035". Similar figures for stainless steel were 0.065" wall, 16 f.p.i. and 0.050" fin height. Later proposals were made to increase the fin density, such as to 26 f.p.i., for titanium, while decreasing the fin height to about 0.025" and reducing the wall thickness under the fin to about the same value. The last noted parameters increased the ratio of the outside tube surface area to the inside area as compared to the parameters formerly used. The aforementioned later proposals are at least generally embodied in U.S. Pat. No. 4,366,859 issued to John M. Keyes. The Keyes patent emphasizes that fin heights should not exceed 0.033" for titanium, or 0.045" for stainless steel, and argues that "fin splits" will occur if these heights are exceeded. The Keyes patent shows the tubing as being finned on a mandrel having a single diameter work surface against which the tube is forced by one or more arbors carrying single sets of discs.
Another patent related to the finning of difficult fin materials is Laing et al, U.S. Pat. No. 3,795,125 which discloses a method of forming fins with a height of at least 0.100" on stainless steel tubes. The fins are formed in two completely separate finning operations through separate sets of discs with differing contours. The second finning operation produces both a substantial increase in the fin O.D. and a decrease in its R.D. but cannot be performed without an intermediate annealing operation. The technique is time-consuming and costly. Also, it is not very practical when making the vast majority of tubes which require intermediate unfinned lands and plain ends due to the fact that there would be a non-predictable varying amount of stretch of the tube between the separate finning passes. This situation would make it practically impossible to produce lands positioned within currently accepted dimensional specifications.
Two patents relating to the finning of easy to fin material such as copper are U.S. Pat. No. 2,868,046 to Greene and U.S. Pat. No. 3,383,893 to Counts. Each shows a disc arbor with spaced sets of discs with the discs all being of different contours.